Enameling steel



3,193,417 ENAMELING STEEL Michael C. Kopehah, Chicago, Ill., assignor to Inland Steel Company, 'Chieago, ill, a corporation of Delaware No Drawing. Filed Dec. 5, 1962, Ser. No. 242,352 12 Qiaims. (6i. 148-31} This invention relates to a novel and improved enameling steel and to an enameled article made from the same.

For the most part, the material known as enameling iron (a low carbon, low manganese rimmed steel) has heretofore been the major ferrous base material supplied to the enameling trade because of its enamel adherence properties and its acceptable sag or warpage resistance in the usual enamel firing temperature range, viz. 1400- 1600 F. However, enameling iron is subject to certain surface defects during enameling, and numerous proposals have been made for the elimination of defects in the enamel surface caused by such wel -known phenomena as boiling, reboiling, blistering, and fish scaling.

Certain of these defects have been attributed to the evolution, at the enamel firing temperature, of carbonaceous gases resuting from the reaction between iron oxide and carbon in the ferrous metal base. To overcome this problem, it is customary to use a two-coat enameling procedure, i.e. a ground coat and a cover coat, in rder to obtain a satisfactory enameled article. Consequently, much effort has been devoted to the problem of overcoming such gas evolution by suitable modifications of the composition of the ferrous metal base so that a single coat of enamel can be used without a ground coat. For example, carbon-stabilizing additions of titanium or the like have been employed in order to tie-up or fix the carbon. This practice, while highly effective, is relatively expensive and has enjoyed somewhat limited commercial use.

Decarburization of ferrous shat material, such as ordinary rim steel, is also known to be effective in eliminating enamel surface defects attributable to gas evolution by carbon reactions. Moreover, decarbur-ized enameling steel has sag or warpage resistance comparable to that of enameling iron, and its enamel adherence properties are also equivalent to those of enameling iron, particularly for two-coat enameling applications. Decarburized enameling steel has the further advantage that the deca-r-burizing step also involves a single step heat treatment which can be regulated to provide either light-drawing or deep-drawing properties, whereas enameling iron usually must be double heat treated to meet deep-drawing requirements.

With the relatively recent availability of commercial open coil annealing equipment, it has now become economically feasible to produce deca-rburized enameling steel and consequently this product is the subject of considerable study and evaluation at the present time. However, a serious problem which has been encountered in the use of de-carburized enameling steel is enamel chippage on loadbear-ing enameled articles. It appears that the mechanism of the chippage is as follows: (1) the decarburized steel is subjected to a critical degree of straining during drawing or other fabrication steps in the formation of the load-bearing member; (2) as a result of this critical strain, a coarse recrystallized grain structure develops during the enamel firing step at 1400-1600 R; and (3) the coarse grain structure results in a substantial loss of yield strength so that plastic deformation of the load-bearing member occurs under an applied load, thereby cracking off the enamel.

Accordingly, it is a primary object of the present invention to provide novel and improved means for retarding or eliminating the above-described coarse grain formation and loss of yield strength in a decarburized enameling steel.

or 1 ce A further object of the invention is to provide a novel and improved decarburized steel and a novel and improved enameled article made from said steel.

Another object of the invention is to provide a novel and improved decarburized steel that can be utilized for enameling by either a single coat process or a two coat process.

An additional object of the invention is to provide a novel and improved decarburized enameling steel, characterized by good ductility and by substantial retention of yield strength after being subjected to strain and high temperature, as in the formation of an enameled article.

Briefly described, the foregoing objects of the invention are realized by the provision of an aluminum killed, rephosphorized, decarburized steel. As ereinafter described in detail, the aluminum content of the steel is about .10 wt. percent max., the phosphorus content is from about .015 wt. percent to about .08 wt. percent, and the carbon content is less than about .01 wt. percent.

A ferrous base material suitable for use as enameling stock should meet the following combination of requirements:

(1) The product must have good ductility suitable for deep-drawing in its as-shipped or temper rolled condition.

(2) The product must be sufiiciently resistant to grain growth after drawing or other cold working and subsequent enamel firing so as to avoid loss of structural strength.

(3) The product must have acceptable enameling characteristics, i.e. good enamel adherence, freedom from enamel surface defects, good sag or warpage resistance, and normal pickle loss characteristics.

In the fabrication of enameled parts wherein the ferrous base material is subjected to drawing to form the part, between about 3% and about 20% elongation can be expected in many of the drawn areas. It has been found that this range of elongation constitutes a critical degree of strain for decarburized steels such that in subsequent enamel firing (e.g. at 1500 F. for 5 min.) a considerable grain growth occurs in the critically strained areas with consequent loss of structural strength as evidenced, for example, by :a marked reduction in yield strength. Although decarburized steel is desirable for use an enameling stock because of its good enameling properties, as heretofore described, it is obviously necessary to overcome the grain growth problem in order to meet the combined requirements for an acceptable enameling base material. Furthermore, in solving the grain growth problem, the ductility of the material must also be preserved so that it is suitable for use in either light or deep drawing applications.

The above-described grain growth problem and loss of strength in decarburizcd enameling steel can be partially counteracted by the addition of phosphorus to the steel, the phosphorus presumably acting as a ferrite strengthener. However, the addition of phosphorus to the steel also results in a considerable loss of ductility so that rephosphorized decarburized enameling steel is not suitable forl deep drawing applications and, therefore, has limited uti ity.

I have discovered that if aluminum killed steel is used as the starting material instead of the usual rim steel, then by rephosphorization and decarburization I can obtain a product having the desired combination of ductility, resistance to grain growth, and favorable enameling properties. Aluminum killed steel has a characteristic elongated or pancake grain structure which imparts good ductility or drawability to the steel. This is not the case with other killed steels such as silicon killed steel. However, prior to my discovery it could not be predicted that this desirable grain structure and favorable drawing qual- 3 ities of aluminum killed steel would be retained to a re markable degree after decarburization. Thus, by using aluminum killed steel as the initial material before decarburization, I have found that the benefits of phosphorus addition in improving the strain-anneal characteristics of the product can be realized without seriously detracting from the ductility and drawing qualities of the initial material. In fact, as the experimental data below will show, the combined effect of' aluminum and phosphorus provides an unexpected and disproportionate improvement in the strain-anneal characteristics of the steel which could not be predicted froma consideration of the effect of either element alone. Moreover, rephosphorization has the additional advantages of improving the pickling characteristics and the enamel adherence properties of the steel.

In summary, myinvention involves the combined effects of decarburization, rephosphorization, and alu'rni- I 'num'killing. As hereinbefore described, decarburization overcomes enamel surface defects attributable to gas evolution by reaction of carbon in the steel. Rephosphorization, particularly within the preferred range of phosphorus content, results in marked resistance to grain growth during subsequent cold working and enamel firing with consequent retention of structural strength. The presence of sufiicient aluminum to obtain a fully killed steel oifsets the detrimental effects of phosphorus and insures the desired ductility and drawing qualities of the steel. Moreover, the addition of phosphorus to the steel enhances its enameling characteristics. The combined effect of these three factors is such that a highly acceptable enameling steel is obtained in a novel and im- "proved manner.

More specifically, the present invention affords an enameling steel having: (a) good enameling characteristics; (b) good drawing properties in the as-shipped or temper rolled condition, e.g. .2% offset yield strength of 22,000 to 26,000 p.s.i., ultimate strength of 44,000 to 4 6,000 p.s.i.,

and 40 to 44% elongation in 2 inches; and (c) good strainanneal characteristics, e.g. .2% offset yield strength of 20,000 to 25,000 psi. after 320% elongation followed by heating under enamel firing conditions.

In the manufacture of enameling steel in accordance wit-h the present invention, a medium or low carbon steel is utilized as the starting material and the usual well 'known aluminum deoxidizing practices may be followed.

. quired for this purpose, and usually from about .02 to about .08 wt. percent will sufiice. The required amount of phosphorus is also conveniently added in the ladle or in the mold or'bot-h, preferably as ferrophosphorus. As described in more detail below, the amount of added ferrophosphorus should be suflicient to provide a phosphorus content in the final product of from about.0l5 wt. percent to about .08 wt. percent. However, a phosphorus content near the lower end of this range may not impart adequate structural strength for deep drawing applications. Accordingly, in order to realize the full benefits of the invention the phosphorus content should be from about .035 wt. percent to about .08 wt. percent.

Sulfur .040 max., preferably .030 max. Aluminum .10 max., preferably .02.08. Balance Essentially iron.

It will be understood that the foregoing steel composition will contain only the usual residual amount of silicon (.002 wt. percent max.) characteristic of an aluminum killed steel.

Decarburization, in accordance with the presentinvention, is accomplished by contacting the aluminum killed rephosphorized steel with a gaseous decarburizing atmosphere at an elevated temperature and for a time sufiicient to effect the desired degree of carbon removal. This dccarburizaiton step may be carried out by any suitable technique, but at the present time open coil annealing has been found to be themost practical and convenient Way of accomplishing the desired result. In open coil annealing, the strip is first coiled with a spacer, such as a nylon cord, interposed between each lap, and the spacer is later tain a reducing gas, such as CO-or H and a controlled Thereafter, the ingots are processed in the usual sequence 7 Carbon .06.l0. Manganese .60 max., preferably .30-.40. Phosphorus 015-.08, preferably .035-.08.

amount of water vapor. The relative amounts of reducing gas and water vapor are regulated so that the atmosphere is non-oxidizing to the steel at the elevated temperature of the decarburizing step, and the time, temperature and atmosphere composition are correlated to achieve the desired degree of carbon removal. The interdependence of the foregoing factors is well understood in the art, and for the sake of convenience the following literature references may be. consulated for a discussion of the principles involved: Jenkins, IvorControlled Atmospheres for the Heat Treatment of Metals, Chapman and Hill, Ltd., 1951, p. 2661f. e

Hotchkiss, A. Webber, H. M.Protective Atmosphere's, Wiley and Sons, Inc., New York, 1953, p. 9 ff. Low, J. R., Jr.; Gensamer, M.--Aging and the Yield Point in Steel, Trans. A.I.M.E., Iron and Steel Division, vol. 158, 1944, p. 207 ff.

Suitable decarburizing atmospheres which may be used in practicing the invention include such commercially available atmospheres as DX gas (comprising C0, C0 H and N or HNX gas (comprising 6l4% H the balance N with added water vapor in either case. Other examples include astraight hydrogenatmosphere or a mixture of hydrogen and nitrogen with added water vapor in either case.

The steel strip is contacted with the .decarburizing atmosphere at a temperature offrom about 1'l00 F. to about 1500? F. (preferably from about l275 F. to about 1350 F.) for the required period of time to reduce the 'carbon content of the steel to not more than about .01 wt. percent and preferably to not more than about .005 wt. percent. The time may range from as low as about 4 "hours to as long as about 40'hou'rs depending upon the initial carbon content of the steel, the temperature, and the composition of the decarburizing atmosphere. In general, increasing the hydrogen content of the decarburizing atmosphere allows an increase in the permissible amount of water vapor without rendering the atmosphere oxidizing to the steel, and thereby increases the reaction rate and decreases the time required to attain the desired low carbon level.

The resultant product following the decarburization step has an'analysis substantially the same as heretofore specified for the fully aluminum killed and rephosphorized steel except that the carbon content has been reduce to .01

wt. percent max., and preferably .005 wt. percent max. After the decarburization step, the enameling steel is temper rolled (1%) in the usual manner and is ready for shipment to the enameler. The steel is fabricated by drawing or the like in the customers plant and the fabricated part is pickled, enameled, and fired using either the conventional two coat or a single coat operation.

The principal beneficial effect of rephosphorization is the retention of yield strength by retarding coarse grain formation during the enamel firing step after the steel has been subjected to critical straining or cold working, such as by drawing or the like. The elongated grain structure and ductility of the aluminum killed steel are also retained in large measure in spite of the brittling tendency of phosphorus. However, the ductility and drawing qualities of the steel may be further enhanced by prolonging the soaking period at an elevated temperature during the latter portion of the decarburizing cycle. For example, in the present invention a typical decarburizing cycle using HNX gas comprises a heating period of 3 /2 hours to bring the steel to a temperature of 1185-1360 F., a decarburization period of 11% hours and a soaking period of 4%. hours. By prolonging the soaking period of this cycle to about 6 hours, a further improvement in ductility of the final product may be obtained, especially for deep drawing applications.

For purposes of further illustrating the invention, but not by way of limitation, the following experimental data are presented.

A series of test specimens were prepared for the purpose of simulating the cold working and enamel firing conditions experienced in various end uses of enameling steel. Each test specimen was a rectangular section 9 and the enamel adherence was evaluated by the standard impact test.

The comparative strain characteristics and enameling properties of several steels made in accordance with the present invention were determined along with the corresponding properties for a normal grade of decarburized rim steel, a decarburized rephosphorized rim steel, a decarburized aluminum killed steel, and conventional enameling iron. The analyses of the various specimens were as follows (wt. percent):

Table I Specimen Description 0 Mn P S Al Deearb. rim steel 002 .35 009 023 Deearb. rephos. rim steel 002 .38 040 020 Decttrb. Al killed Steel 002 38 009 030 030 Decarb. Al killed rephes.

st e1. .003 .38 .023 .028 .037 002 .38 040 .025 031 002 39 048 030 032 Enameling iron- 015 06 012 021 T able 11 Values after straining and heating Tensile Initial Pickle Specimen Property 1 Value Loss,

3% 7% 10% 12% 15% grrL/sq. it

Yield 22 25. 8 I Ultimate 44 41. 3 1. 5-3. 5

Elongation 44 42 Y'eld 29 1s 14 v II 38 30 7. 5

26. 1 26. 9 15. III 39. 4 29. 9 34. 3. 1

38 30 40 J'Y' d 30. 2 31. 5 11. IV 40. 3 39. 6 30. 6. 5

l 43 4e 48 Y 26. 9 25. 4 19. V 41. 3 38. 5 37. 9. 0

. 35 34 37 29. 7 31. 1 23. 1 VII 47. 9 49. 8 48.8 3-5 Elongation 35 32 19 46. 1

Yield strength (0.2% offset), p.s.i. X 1000; ultimate strength, p.s.i. X 1000; elonga tion percent; in 2 in.

inches by inch with a thickness of about 20 gauge. For each material a plurality of specimens were strained to various degrees of elongation between 3% and 20% in a standard tensile machine. The strained specimens were then heated for about 5 minutes at 1500 to 1550 F., cooled, machined to standard size, and subjected to tensile testing.

In addition to measuring the tensile properties of the strained and heated materials, duplicate unstrained test specimens were also evaluated for pickling characteristics and enameling properties. The test for pickling characteristics comprised immersing the specimen for 15 minutes in 9 wt. percent aqueous sulfuric acid at 170 F. and determining the total weight loss in gm./sq. ft. The pickled specimens were then spray coated with a single coat of white enamel and fired for 5 minutes at 1550 F. The appearance of the enameled specimens was noted It will be seen that the initial tensile properties of decarburized rim steel (specimen I) are satisfactory but the strain-anneal characteristics are poor as shown by the low yield strength at 10% nd 20% elongation. Moreover, the low pickle loss characteristic of ordinary decarburized steel is a disadvantage to the enameler since it increases the total time required for pickling prior to enameling.

Rephosphorization alone (specimen II) of the decarburized steel affords a slight improvement in strain-anneal characteristics as evidenced by yield strengths at 7%, 12%, and 20% elongation which are somewhat higher than the corresponding values for specimen I. However, the initial properties of the rephosphorized decarburized steel are unsatisfactory because of poor ductility as indicated by the high yield strength of 29,000 p.s.i. and the low elongation of 35%.

Specimen 111 comprising the aluminum killed decarburized steel has initial tensile properties and drawability equivalent to the decarburized rim steel but. the strainanneal characteristics are also poor as seen-from the low yield strength at'12%, and elongation. 7

Specimens IV, V, and VI comprise fully aluminum killed, rephosphorized, decarburized steels in accordance with the present invention. Specimens V and VI show remarkably good strain-anneal characteristics even at severe 'elon'ga'tions of 1220%, as compared with specimens I, II, and III. Thus, it will be seen that-the combined eifect of aluminum andphosphorus is more than additive and much greater than would have been predicated from the results on specimens II and III. The initial ductility of specimens V and VI is quite acceptable although the elongation is slightly less than specimen 1. However, a somewhat longer soaking period at the end of the decarburizing cycle would provide additional annealing and raise the elongation to substantially the 4'4% value of specimen I. Specimen IV having a lower level of rephosphorization displays excellent initial tensile properties but the strain-anneal characteristics at severe elon: gation are such that the usefulness of the steel would be limited primarily to light drawing applications. It will be noted that specimens IV, V and VI also have excellent pickel loss thereby greatly simplifying and shortening the pickling step performed by the enameler. V

The data for specimen VII are included for completeness and show the much poorer initial ductility and lower pickle loss of enameling'iron as compared with the steelsof the present invention. I p In each case the enameling tests showed that the specimens had acceptable and approximately equivalent enamel adherence and surface appearance. The enameling characteristics were not impaired by the presence of aluminum and phosphorus in specimens IV, V, and VI.

From the foregoing, it will be seen that my invention provides a novel and improved enameling steel having the excellent enameling properties and drawing qualities of ordinary decarburized steel but without the disadvantageous strain-anneal characteristics of ordinary decarburized steel.

1. An aluminum killed rephosphorized, decarburized enameling steel containing about .01 wt. percent max. carbon, from about .015 to about .08 wt. percent phosphorus, and about .10 wtupercent max. aluminum, the balance being essentially iron with the usual impurities of medium or low carbon steel.

2. An aluminum killed, rephosphorized, decarburized enameling steel containing about .005 wt. percent max. carbon, from about .035 to about .08 wt. percent phos phorus, and from about .02 to about .08 wt. percent aluminum, the balance being essentially iron with the usual impurities of medium or lowcarbon steel.

'3. An aluminum'killed, rephosphorized, decarburized enameling steel containing about .01 wt. percent max. carbon. about .60 wt. percent max. manganese, from about .015 toabout 0.8 wt. percent phosphorus, about .040 wt. percent max. sulfur, and about .10 wt; percent max. aluminum, the balance being essentially iron. f

4. An aluminum killed,-rephosphorized, decarburized enameling steel containing about .005 wt. percent max. carbon, from about .30 wt. percent to about .40 wt. percent manganese, from about .035 to about .08 Wt. percent phosphorus, about .030 wt; percent max. sulfur, and from about .02 to about .08 wt. percent aluminum, the balance being essentially iron.

5. A'vitreous enameled article formed from the steel of claim 1.

6. A vitreous enameled article formed from the steel of claim 2.

7. A vitreous enameled article formed from the steel of claim 3.

8. A vitreous enameled article formed from the steel of claim 4.

9. An improved enameling stock suitable for both light and deep drawing and'characterized by resistance to grain growth and retention of yield strength upon strainf'inginthe range of 3% to 20% elongation followed by heatingto enamel firing temperature, consisting essentially of a fully aluminum killed, rephosphorized, decarburized steel containing about .01 wt. percent max. carbon, from about .035 to about .08 wt. percent phosphorus,

and about .10 wt. percent max; aluminum, the balance being essentially iron with the usual impurities of medium or low carbon steel.

10. The steel of claim 9 further characterized in that the carbon content is about .005 wt. percent max.

11. An improved enameled article having a steel base strained to the extent of from 3% to 20% elongation prior to enamelfiring and characterized by retention of yield strength, said steel base consisting essentially of a fully aluminum killed, rephosphorized, decarburized steel containing about .01 wt. percent maxcarbon, from about .035 to about .08 wt. percent phosphorus, and about .10

-wt. percent max. aluminum, the balance being essentially iron with the usual impurities of medium or low carbon r steel.

12. The article of claim 11 further characterized in that the carbon content is about .005 wt. percent max.

References Cited by the Examiner UNITED STATES PATENTS L. RECK,Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Non 3,193,417 July 6, 1965 Michael (1., Kopchak It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, lines 12 and 13, for "decarburizaiton" read decarburization line 37, for "consulated" read consulted column 6, Table 11, under the column heading "7%", line 8 thereof, for "29.9" read 39,9 column 8, line 4, for "0.8" read .08

Signed and sealed this 21st day of December 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

4. AN ALUMINUM KILLED, REPHOSPHORIZED, DECARBURIZED ENAMELING STEEL CONTAINING ABOUT .005 WT. PERCENT MAX. CARBON, FROM ABOUT .30 WT. PERCENT TO ABOUT .40 WT. PERCENT MANAGNESE, FROM ABOUT .035 TO ABOUT .08 WT. PERCENT PHOSPHORUS, ABOUT .030 WT. PERCENT MAX. SULFUR, AND FROM ABOUT .02 TO ABOUT .08 WT. PERCENT ALUMINUM, THE BALANCE BEING ESSENTIALLY IRON. 